TiB2 rod, and method of fabrication and use

ABSTRACT

A TiB 2  rod, and a method of fabrication therefore are is provided. It may be used to producing a TiB 2  coating on the tip surfaces of a resistance spot welding (RSW) electrode, such as may be of a type for welding applications in automotive, and electronic industries. There may be a rod of material for coating the tips of welding rods that includes TiB 2  ceramic having good electrical and thermal conductivities and a high melting temperature. The new TiB 2  rod may be used to produce a TiB 2  coating on an electrode tip surfaces by ESD, which may tend to be low in cost and which may tend to produce a higher electrode life as compared to an embodiment of TiC coating.

AREA OF APPLICATION OF INVENTION

The TiB₂ coating rod, such as may be used to provide by an electrospark deposition (ESD) coating layer on a welding electrode may be used in the field of Resistance Spot Welding (RSW) such as may be used, for example, in applications in industries such as the Automotive and Electronics industries.

TECHNICAL BACKGROUND

With the progress in the automotive industry, regular sheet steels can not meet the corrosion-resistant requirement. Various coated sheet steels, such as Zinc coated steels for example, may be found in an increasing number of applications in automobiles. However, the resistance spot welding of zinc coated steels may necessitate a higher welding current or longer weld time as compared to uncoated bare steel, because of the lower melting point of Zn and hence the reduced contact resistance as compared to bare steels. This may tend to increase the operating temperature of the electrodes. Under the combined effects of heat and pressure, the electrodes may tend to react with the Zn coating to form low melting point alloys. The formation of these alloys may tend to hasten degradation of the electrodes. Therefore, researchers have tried to find methods to improve electrode base materials, and to modify the electrode surface conditions. The surface modification methods may be more effective and may be less expensive. A coating layer, which may be a ceramic coating layer, such as may be deposited on electrode surfaces may have a high melting point, high electrical conductivity, high strength and low reactivity with Zn. Such a coating may tend to stop or minimize the reaction between the electrodes and the Zn coating, and hence may tend to improve electrode tip life, and may tend to reduce the sticking tendency between the electrode and the workpiece. As such, there is a need for an advantageous coating material for application on the electrode tip surfaces.

The major surface modification methods include PVD (physical vapor deposition) and ESD (Electrospark Deposition). Others have made observations in this field. For example, Gobez [1] applied Co, Ta, Ni, TiN or Mo on CrZrCu electrode, and the results indicated that the first three coatings decreased the electrode tip life but the last two improved the wear resistance of the electrode. Studdon [2] at Wollongong University used TiN on Zn—Al coated steels, and the results indicated 70% improvement in electrode tip life. Ashcroft et al. [3] applied CrN using PVD on Zn—Al coated steels, the results indicated that the welding current was reduced by 10% but no improvement in electrode life was experienced. But the electrode life was stable at ±7% compared to ±40% with electrode coating. TiC coating was used to improve electrode tip life in microwelding and the results showed the TiC coating by EDS improve the tip life by 200% (from 600 to 1200) because the TiC coating reduced the local bonding between electrodes and Ni-coated steel and the deformation of the electrodes. The TiC coating by Huys Industries can also improve the electrode sticking resistance. It improved the CuCrZr electrode life in RSW of Zn-coated steels for automotive applications by 250% (from 400 for the uncoated electrodes to 1000 for coated electrodes).

DETAILED DESCRIPTION

By way of general overview, this description pertains to a TiB₂ rod and its fabrication for the use of producing TiB₂ coating on resistance spot welding (RSW) electrode tip surfaces for welding applications in automotive, and electronic industries. There may be a rod of material for coating the tips of welding rods that includes TiB₂ ceramic having good electrical and thermal conductivities and a high melting temperature. The new TiB₂ rod may be used to produce a TiB₂ coating on an electrode tip surfaces by ESD, which may tend to be low in cost and which may tend to produce a higher electrode life as compared to an embodiment of TiC coating.

In one embodiment, there may be a TiB₂ coating rod for the use of coating on RSW electrode tip surface by ESD. That rod may have a chemical composition of, in wt %, 12-24 Ni, 4-10 Mo, 0.15-1.0 W, 0.15-1.0 Co, and balance TiB₂. This rod may be used for electrospark deposition (ESD) on RSW electrode tip surfaces in automotive and electronics industries.

An aspect of this invention also covers the fabrication of the TiB₂ rod for the use of producing TiB₂ coating on RSW electrode tip surfaces. The rod may be fabricated as follows:

-   -   1. Powders of Ni, Mo, W, Co and TiB at the above mentioned         weight percentage are mixed uniformly and milled for 24-96         hours.     -   2. Binder is then added and the mixture is formed into a green         rod.     -   3. The rod is debinded and sintered under 10 MPa to 30 MPa         pressure in a controlled atmosphere at 1500-1900° C.

Debinding may refer to heating or baking the green rod for a period of time at moderate, non-sintering temperatures prior to sintering.

This TiB₂ rod may be used to coat a layer using ESD onto the RSW electrode tip surface. The coated layer may typically be of the order of 10-25 μm in thickness, and may tend to be very stable. The coated layer may tend to improve the tip life of conventional electrodes (such as Cu—Cr—Zr), without tip dressing. In one embodiment the improvement may be about by 280%, or as much as 1400 cycles, without tip dressing. The TiB₂ layer may tend to improve tip life, and may tend to be relatively inexpensive.

In a preferred embodiment, the rod may have a rod composition (in weight percentage) of: 15-20 Ni, 4-8.5 Mo, 0.5-1.0 W, 0.3-1.0 Co, and balance TiB₂. It may be milled for 48-72 hours. It may be sintered in a temperature range of 1650-1800° C.

A few experimental examples will be given to illustrate, using tables, the performance of the TiB₂ coating on electrode tip surfaces using embodiments of TiB₂ rods, as a comparison to TiC coating.

Table 1 shows the chemical compositions (all in weight percentages) of four samples of different TiB₂ rods as compared to the TiC rod. It also indicates the average times needed to coat a domed electrode of 16 mm in diameter and 8 mm in radius. It is shown that all four samples of the TiB₂ rods are similar to TiC rods and may be used as coating rods for the ESD coating of RSW electrode surfaces.

Table 2 shows a comparison of the tip life of the coated electrodes with the four TiB₂ and TiC rods in RSW of 0.8 mm thick Zn-coated steel. It indicates that the electrode life using the TiB₂ coating by the four TiB₂ rods is improved as compared to the TiC coated electrode. The following are the details of the examples:

-   -   Sample No. 1 Mixing uniformly of powders of 736 g TiB₂, 185 g         Ni, 61 g Mo, 9 g W, and 9 g Co and then milling the mixture for         52 hours. The mixed powders are made into green rods by adding         83 g binder and then debinded at 220° C., and then sintered         under 10 MPa pressure in a controlled atmosphere at 1710° C.     -   Sample No. 2 Mixing uniformly of powders of 756 g TiB₂, 184 g         Ni, 42 g Mo, 10 g W, and 8 g Co and then milling the mixture for         60 hours. The mixed powders are made into green rods by adding         75 g binder and then debinded at 270° C., and then sintered         under 10 MPa pressure in a controlled atmosphere at 1780° C.     -   Sample No. 3 Mixing uniformly of powders of 748 g TiB₂, 156 g         Ni, 85 g Mo, 6 g W, and 5 g Co and then milling the mixture for         52 hours. The mixed powders are made into green rods by adding         91 g binder and then debinded at 190° C., and then sintered         under 10 MPa pressure in a controlled atmosphere at 1680° C.

Sample No. 4 Mixing uniformly of powders of 757 g TiB₂, 162 g Ni, 71 g Mo, 7 g W, and 3 g Co and then milling the mixture for 52 hours. The mixed powders are made into green rods by adding 86 g binder and then debinded at 280° C., and then sintered under 10 MPa pressure in a controlled atmosphere at 1750° C. TABLE 1 Composition of the rods and coating time Example TiC for 1 2 3 4 comparison TiB2 73.6 75.6 74.8 75.7 Ni 18.5 18.4 15.6 16.2 Mo 6.1 4.2 8.5 7.1 W 0.9 1.0 0.6 0.7 Co 0.9 0.8 0.5 0.3 Coating time 57 59 61 56 65 (Seconds)

TABLE 2 Average electrode tip life 0.8 mm thick Zn-coated steel TiC 1000 Example 1 1400 Example 2 1500 Example 3 1550 Example 4 1450 

1. A TiB₂ rod to for use in coating an RSW electrode tip surfaces using EPD, said rod including (in weight percentage): 12-24 Ni, 4-10 Mo, 0.15-1.0 W, 0.15-1.0 Co, and balance TiB₂.
 2. The rod of claim 1, including (in weight percentage): 15-20 Ni, 4-8.5 Mo, 0.5-L1.0 W, 0.3-1.0 Co, and balance TiB₂ in claim
 1. 3. A method of fabricating the rod of claim 1, said method including the steps of obtaining powders of said Ni, Mo, W, Co and TiB₂; mixing said powders evenly; milling said powders for 24-96 hours; adding 2-10% binder; debinding 100-300° C.; and sintering under 10 MPa to 30 MPa pressure in controlled atmosphere at 1500-1900° C.
 4. A method of fabricating the rod of claim 2 wherein said method includes the steps of obtaining powders of said Ni, Mo, W, Co and TiB₂; mixing said powders evenly; milling said powders for 48-72 hours; adding 2-10% binder; debinding 100-300° C.; and sintering under 10 MPa to 30 MPa pressure in controlled atmosphere at 1650-1800° C. 